So far Innercool has completed over 60 patients for its TCAS clinical study in neurosurgery. All of the experimental patients and control were intubated and had an esophageal temperature probe for temperature monitoring. Monitoring temperature in the distal esophagus has been shown to be extremely reliable for monitoring continually core temperature, and for providing temperature feedback for controlling the induction and maintenance of hypothermia.
As previously mentioned, control algorithms are sometimes used to control the rate at which heat is extracted from the body by the catheter. These algorithms may be embodied in hardware, software, or a combination of both. The gain factor employed by such algorithms is dependent on the effective thermal mass of the body or organ being cooled. Thus, it is important to determine the effective thermal mass so that an appropriate gain factor can be calculated for the feedback control algorithm.
The mass of the body (organ or whole body) being cooled can be estimated by relating the power removed by the catheter to the power lost by the body.
The power removed by the catheter may be expressed as follows:Pcatheter=McfΔT  (1)Where M is the mass flow rate of the fluid circulating through the catheter (typically measured in terms of cc/s), cf is the heat capacity of the fluid, and ΔT is the temperature difference between the working fluid as it enters the catheter and as it exits the catheter. Accordingly, Pcatheter can be readily calculated by measuring the mass flow of the circulating fluid and the temperature difference between the working fluid as it enters and exits the catheter.
The power removed by the catheter as determined by equation (1) may be equated to the power that is lost by the patient's body:Pcatheter=mcb∂T/∂t  (2)Where Pcatheter is now the power lost by the patient's body and has the value calculated by equation (1), m is the effective thermal mass of the body being cooled, cb is the heat capacity of the body, and ∂T/∂t is the change in temperature per unit time of the mass being cooled.
Accordingly, the effective thermal mass of the body being cooled is:m=Pcatheter/(cb∂T/∂t)  (3)Since all the variables in equation (3) are either known or are measurable, the effective mass can be determined.
The mass calculated in this manner is an effective thermal mass that represents the portion of the body from which power is removed (i.e., the portion of the body that is cooled). The temperature change in equation (3) represents the temperature change of the portion of the body being cooled. For example, if whole body cooling is to be performed, the change of the core body temperature may be measured to calculate mass in accordance with equation (3). In general, for whole body cooling, if the patient is vasoconstricted, the effective mass may represent about 50% of the total body mass. If the patient is vasodilated, the effective mass will be closer to the total body mass.
Alternatively, if only a selected organ such as the brain is to be cooled, then the temperature change that will be used in equation (3) would be the temperature change of the organ, assuming of course that the organ can be at least briefly considered to be largely thermally isolated from the remainder of the body. In this case the effective mass that is determined would be comparable to the mass of the organ. If the selected organ to be cooled is the brain, for example, the catheter is placed in the common carotid artery, the internal carotid artery, or both. The temperature changed used in equation (3) will be measured by inserting a temperature sensor into the brain or via a tympanic membrane sensor, both of which are commercially available.